1. Field of the Invention
The present invention relates generally to the fields of ophthalmic instruments that are used to examine the eye. More specifically, the present invention relates to ophthalmic examination instruments that measure and characterize the properties of the eye with high accuracy of their localization.
2. Description of the Related Art
The eye is not an ideal optical system and, therefore, different reference axes can be used to describe its properties in a 3D space. These properties cannot be comparable unless they are calculated with respect to the same reference axis. Four axes are used: optical axis, pupillary axis, line of sight, and visual axis. Similarly to the perfect optical systems, the optical axis of the eye crosses the optical centers of two main optical components, the cornea and the crystalline lens.
In the first steps of aberrometry development, the sub-commission of the Optical Society of America (OSA) working group recommended to the ophthalmic community to use the line of sight as the reference axis for the purposes of calculating and measuring the ocular optical aberrations (1). One of the arguments was that the center of the pupil can be easily designated. In parallel, the visual axis was often taken for the reference, especially in corneal topography, because it was easily identified using Purkinje reflections from the anterior cornea. Having different reference axes in corneal topography, in optical coherence tomography, in aberrometry, in refractometry, etc., caused difficulties when trying to integrate the data from separate instruments. The surgeons discovered worse vision correction in patients with higher angle kappa, i.e., higher angular distance between the pupillary axis and visual axis.
The cause of such statistics was the usage of an incorrect reference axis when performing the surgery. Usually, the eye is pupillary centered during the surgery, meanwhile, the imaging in the eye is taking place through the nodal point which is the case when measuring the aberrations. The major reason for this is the current misconception among many laser companies and surgeons that ablations are centered on the entrance pupil centre instead of the visual axis. In eyes with a large angle kappa, centering the ablation on the entrance pupil centre will result in the creation of a new vertex, whereas the natural vertex is maintained if the ablation is centered on the corneal vertex (2).
In the US Patent Publication 20100114076, a method of performing refractive laser eye surgery on a human eye is disclosed where the ablation pattern is centered along the visual axis, rather than along the line of sight. A wavefront, either ocular, corneal or a combination thereof, is generated by a wavefront sensor centered along the line of sight. Then, an analysis pupil is determined that is centered along the visual axis at the point of intersection with the cornea. The reconstruction of the measured wavefront is done through a least squares fit of a series of slopes from the measured wavefront and/or through the transformation of aberration coefficients. Finally, an ablation pattern, or a lenticule generation pattern, to be performed by a refractive laser centered on the corneal intersect of the visual axis, is produced in accordance with the reconstructed wavefront. However, there is no disclosure on how a new, analysis pupil is determined nor how the point of corneal intersection is found.
In practice, the visual axis is determined subjectively. The patient is asked to orient his sight to overlay two targets, near and far, positioned on the optical axis of the measuring instrument. With this orientation of the eye, measurement of eye parameters is made, for example, with a ray tracing aberrometer (3). Simultaneously, an image of the pupil is registered. On this image, the position of the optical axis of the measuring instrument is identified as the center of the set of measuring laser beams. This position is taken also for reference to identify the visual axis of the eye.
A drawback of this approach is in the uncertainty of its subjective procedure. The accuracy depends on a subjective patient's estimate, not on objective measurement. Subjective judgement is difficult because it is impossible for both points to be in focus for the patient. However, objective measurements allow one to determine the point of the corneal intersection by the visual axis, and to take into account a corresponding shift of eye parameters respectively the pupil, and to use these data to measure any other characteristics of the eye, or to correct the refraction map for vision correction by means, for example, of the corneal tissue ablation.
Thus, there is a recognized need in the art for improved methods for determining the trace of the visual axis objectively without depending on a patient's estimates. Particularly, the prior art is deficient in methods for objectively determining the axis of best vision of the eye. The present invention fulfills this longstanding need and desire in the art.